Boron Trifluoride Hardeners

The homopolymerisation of bisphenol A glycidyl ethers via their epoxide and hydroxyl groups can be activated by alkalies especially inorganic alkaline and alkoxides, and by certain acidic substances such as Lewis acids, of wihic boron trifluoride (BF3) is an important example; but the properties of homopolymers are generally considered to be inferior to the addition polymers formed by crosslinking, and there is practically no published information about them. From this is follows that boron trifuoride used purely as a homopolymerisation catalyst for cross linking reaction with both anhydride and amines, especially those reactions involving the slow curing amines. But it would be difficult to establish the extent of catalyzed metathetical cross linking unconfused with catalyzed homopolymerising action.

Boron trifluoride forms chemical complexes with certain compounds, especially amines to give hardeners that are latent in effect until heated. An important one is the BF3 monoethylamine complex, with requires heating to about 95oC for dissociation to occur. This behavior enables one package adhesives with mild curing cycles to be formulated. The complex also has a latent curing effect in association with cross linking hardeners; used with diaminodiphenyl sulphone for example, an addition of one per cent enables the normal curing time to be reduced to about one sixth. However, the remarks at the end of the previous paragraph also apply here.

The nature of the amine affects the reactivity of BF3 amine complexes towards epoxide groups; for example, reactivity decreases in the order ethylamine (primary amine), diethylamine (secondary amine), triethylamine (tertiary amine).

Valuable information the use of boron compounds including BF3 complexes has been given in a paper by Lee and Neville. The use of other boron compounds is mentioned in the next articles.